1. Field of the Invention
The present invention relates to a transparent substrate which includes a natural stuff. More specifically, the present invention relates to a transparent substrate which includes a cellulose fiber having a substituted group.
2. Description of the Related Art
In recent years, much research has been conducted on natural stuffs available for use as a transparent substrate as an alternative to a triacetyl cellulose (TAC) film. While it has been, for example, found that it is possible to make remarkably fine cellulose nanofibers from cellulose fibers by a chemical and/or mechanical treatment, it has further been found that a formed article having robustness and a low linear expansion coefficient can be obtained from a composite material in which such cellulose nanofibers are utilized as an infill (Patent document 1, for example).    Patent document 1: JP-A-2010-116477.
In the case where the formed articles manufactured by a conventional forming method is employed in a practical product, however, there are problems of insufficient transparency, discoloration by heat, low water resistance that causes swelling with water when immersed or dipped in water, dimensional instability when left in an air because of moisture absorption, and large variations in physical properties due to water absorption. The formed article manufactured by a conventional forming method lacks sufficient stability for use as a transparent substrate. In particular, the formed article has not had sufficient durability to be used as a transparent substrate of a protection film for a polarizing plate etc. It has been inevitable to find and establish some new forming processes for solving these problems.